Measuring the flow of gases and liquids through tubing or pipes is required in many application areas. The volume flow through the tubing or pipe is often what is desired to measure. Volume flow meters are however invasive, fragile and often fairly bulky, why it is usual to instead measure the mass flow using mass flow meters. Knowing the pressure and the temperature, a mass flow may be converted to a volume flow by relatively simple calculations. A usual type of mass flow meter is the differential thermal mass flow meter.
A differential thermal mass flow meter has a flow channel in which a heating element and two thermal sensors reside. One of the thermal sensors is situated up-stream compared to the heating element, the other thermal sensor down-stream. As a gas or liquid flows through the flow channel, the heater affects the thermal sensors differently, and a temperature difference between the two thermal sensors occurs. Each temperature difference corresponds to a certain flow rate for a specified gas or liquid. The problem with this setup is that long term use and contamination affects the heat transfer from the gas or liquid to the thermal sensors, as the surface properties of the sensor are changed either by having a layer of particles accumulating thereon or parts of a particle layer flaking off after calibration. This long-term wear thus makes the flow-to-temperature-relation drift, and an erroneous mass flow will be measured.
A further problem with differential mass flow meters is that the distance between the two thermal sensors and the heating element is fixed. The optimal distance between the down-stream arranged thermal sensor and the heating element varies with the mass flow, which leads to a situation in which the mass flow meter will have the highest possible accuracy for only one certain gas or liquid mass flow. A still further problem with differential mass flow meters is that rapid pulsation in the flow is hard to measure. The heat transfer from the heater to the medium (gas or liquid inside the flow channel), and then from the medium to the temperature sensor is not immediate, but is limited by the heat conductivity of the medium as well as the response time in the temperature sensors. This thermal inertia gives a low-pass filtering effect, limiting a measurable frequency of pulsation.
Thus, there is a need for an enhanced differential thermal mass flow meter with a view to overcoming the above problems of differential thermal mass flow meters of the prior art.